Attempting to add weight changes to every rbyd append

This does not work as is due to ambiguity with grows and insertions.

Before, these were disambiguated by seperate grow and attr tags. You
effectively grew the neighboring id before claiming its weight
as yours. But now that the attr itself creates the grow/insertion,
it's ambiguous which one is intended.

lfs_util.h

@@ -100,6 +100,14 @@ extern "C"
 #endif
 #endif
 
+#ifndef LFS_UNREACHABLE
+#ifndef LFS_NO_ASSERT
+#define LFS_UNREACHABLE() __builtin_unreachable()
+#else
+#define LFS_UNREACHABLE() LFS_ASSERT(false)
+#endif
+#endif
+
 
 // Builtin functions, these may be replaced by more efficient
 // toolchain-specific implementations. LFS_NO_INTRINSICS falls back to a more